We are interested in the problem of spatial information processing in the visual system and wish to carry out electrophysiological studies of single neurons. Most complex cells in areas 17 and 18 of the cat have receptive fields consisting of 4 to 7 periodically spaced zones of increased excitability which may allow such cells to carry out a relatively narrow-band spatial frequency analysis of discrete regions within the visual field. Studies of these cells with moving sine-wave gratings have in general confirmed the predictions of the single slit studies. We now wish to determine whether complex cells in the striate cortex of the primate behave similarly and to extend our investigations to areas 18 and 19 and still higher visual areas of the monkey. Our general approach to each cell is as follows: the receptive field and the preferred direction(s) are first approximately determined. Orientation tuning studies using narrow slits moving perpendicular to the test orientation are then carried out to determine the preferred orientation and direction of movement. Directionally sensitive cells are frequently found in areas 17 and 18 but pandirectionally sensitive cells as well as unidirectional and bidirectional cells seem to exist in area 19, and some still higher visual areas, such as the inferotemporal cortex. In directionally sensitvie cells the optimal length and the optimal velocity will be determined as well, and then narrow slits of oPtimal length will be moved across the receptive field in the preferred direction at optimal velocity. Average response histograms to a number of such sweeps will be generated and we will try to determine a "receptive field shape" under such conditions. If the field shapes are periodic, we will determine the spatial frequency selectivity of the cell using sine-wave gratings. If the receptive field shapes are not periodic, extended edges and sine-wave gratings of different spatial frequencies will be used to further test spatial properties. Proposed studies on pandirectional cells will be described elsewhere.